1. Field of the Invention
The present invention relates to photothermographic materials and, particularly, to a photothermographic material favorably used in medical diagnosis, industrial photograph, printing, and COM. Further, the present invention relates to the photothermographic material, suitable for laser exposure and favorably applied to medical images, photographic printing plates or the like, which is excellent in storage stability with passage of time and image storage stability. In particular, the present invention relates to the photothermographic material appropriate for blue-color laser exposure and red to infrared laser exposure.
2. Description of the Related Arts
In recent years, a dry photographic development process is strongly desired in the fields of medical diagnosis and printing plate-making from the standpoints of environmental preservation and space saving. In these fields, digitalization is progressing whereupon a system in which image information is inputted to a computer, stored, optionally processed, transmitted to a place where an image is required by communications, outputted to a photosensitive material by a laser image setter or a laser imager, and the photosensitive material is developed to form an image at the place, is rapidly spreading. As for the photosensitive materials, capability of recording by laser exposure of high illuminance and forming a clear black image having high resolution and sharpness is required. As for such digital imaging recording materials, various types of hard copy systems utilizing a pigment or a dye, such as an ink jet printer and an electronic photographic system, are distributed as an ordinary image-forming system. However, none of the hard copy systems is satisfactory in regard to image qualities (sharpness, graininess, gradation, and color tone) which decide diagnostic performance as an image for use in the medical use and recording speed (sensitivity). Thus none of the hard copy systems has attained a level which can replace a conventional wet-development-type silver salt film for medical use.
On the other hand, thermally developable image forming systems utilizing an organic silver salt are described in, for example, U.S. Pat. Nos. 3,152,904, and 3,457,075 and D. Klosterboer, “Thermally Processed Silver Systems” (see, Imaging Processes and Materials, Neblette, 8th Ed. compiled by Sturge, V. Walworth and A. Shepp, Chap. 9, page 279, 1989). In particular, the photothermographic material comprises a photosensitive layer in which a photosensitive silver halide, a reducing agent, a reducible silver salt (for example, organic silver salt) and, optionally, a toning agent for controlling color tone of silver are ordinarily dispersed in a binder matrix.
When the photothermographic material is heated at a high temperature (for example, 80° C. or more) after being exposed imagewise, a monochromatic black silver image is produced by an redox reaction between the silver halide or the reducible silver salt (functioning as an oxidizing agent) and the reducing agent. The redox reaction is accelerated by a catalytic action of a latent image of the silver halide generated by such exposure. Therefore, the monochromatic silver images are formed in exposed areas of the material. Such photothermographic materials are disclosed in many references other than those described above (see, for example, U.S. Pat. No. 2,910,377 and Japanese Patent Publication (JP-B) No. 43-4924).
In the above-described photothermographic material, a polymer which has a glass transition point in a temperature region lower than that of a thermal development temperature is used as a binder.
As for laser light according to the invention, a gas laser (Ar+, He—Ne, or He—Cd), a YAG laser, a dye laser, a semiconductor laser and the like are ordinarily used. Further, a combination of the semiconductor laser with a second harmonic generating element or the like can also be used. As for light-emission wavelengths, lasers in a wide range of wavelength regions of from a blue region to an infrared region are used. Among other things, an infrared semiconductor laser is low in price and can obtain a consistent light emission whereby it is appropriate for designing a laser image output system which is compact, easy in manipulation, convenient and needs no particular choice of location to be placed in. For this account, infrared sensitivity is required for the photothermographic material and, accordingly, various kinds of studies have been made for enhancing the infrared sensitivity. Infrared spectral sensitization, however, has a problem that it is unstable and liable to be decomposed during a storage period of the photosensitive material to decrease sensitivity thereof whereupon not only enhancement of sensitivity but also improvement of storage stability thereof have been required.
In recent years, a blue semiconductor laser has been developed whereupon, since it becomes possible to perform ultra-fine image recording, attain an increase of recording density and obtain a long-life and consistent output, it is expected that demand for the blue semiconductor laser will be increased and, according, the photothermographic material corresponding thereto is required.
In the photothermographic material, a photosensitive silver halide remains in a film even after development is performed and, at the same time, an undeveloped organic silver salt remains therein. Further, since all materials necessary for development are previously contained in the film and remain therein even after the development is performed, when an image generated by the development is exposed to light or stored at a high temperature, fog is increased or an additional development proceeds whereupon image densities are changed, or silver color tones are changed, thereby causing a serious problem in storage stability of the image.
It is found that, when silver iodide is used as a photosensitive silver halide in the photothermographic material, storage properties of an optical image are improved; however, sensitivity thereof is lower than that in a case of using silver bromide, or silver iodobromide having a low iodine content.
As a way for increasing sensitivity of a silver iodide photographic emulsion, it is recited in scientific literatures and the like that sensitization is performed by addition of halogen receptors such as sodium nitrite, pyrogallol, and hydroquinone to the emulsion, immersion in a silver nitrate aqueous solution, or sulfur sensitization at pAg 7.5 (see, for example, The Journal of Photographic Science, Vol. 8, page 119 (1960), ibid. Vol. 28, page 163 (1980), or Photographic Science and Engineering, Vol. 5, page 216 (1961)). However, a sensitizing effect by these halogen receptors in the photothermographic material, that is a theme of the invention, is very small, thus, extremely unsatisfactory. On this account, development of a technique which is capable of substantially enhancing the sensitivity of the photothermographic material which contains silver iodide having a high iodine content has ardently been desired.
Particularly, in the photothermographic material which uses an organic solvent as a coating solvent, since the organic solvent inhibits adsorption of various kinds of chemical sensitizers to silver halide grains, a sensitization effect can not be fully exerted. Thus, it has been a difficult problem to realize high sensitivity.